This invention relates to a mold for molding a resin which is composed of a fixed mold and a moving mold and used with a stamper being attached to a surface of one of the molds forming a cavity when both molds are closed, a mold for molding, for example, a compact disc, an optical disc, a magnetic optical disc, a laser disc, or the like, and a method of forming a hard film at a part to be contacted with a stamper on the surface forming the cavity for the mold for molding a resin.
In the case of fabrication of a disc-shaped record medium such as a compact disc, an optical disc, a magnetic optical disc, a laser disc, or the like in which music, image, and the like are recorded, a stamper (a mother die) is attached to a surface of one (generally a moving mold) of molds of a resin molding mold composed of a fixed mold and the moving mold forming a cavity when both molds are closed, and a resin is injected with pressure into the cavity, thereby molding the resin and simultaneously transferring projections and depressions on the surface of the stamper.
The fixed mold and the moving mold of the resin molding mold described above are formed made of steel for a resin molding mold and their stamper mounting face and cavity forming faces are subjected to hardening and annealing to improve resistance-to-wear as shown in Japanese Patent Laid-open Publication No. 62-267937 (JP, 62-267937, A).
The stamper mounting face and the cavity forming faces are finished in a mirror face state, thereby increasing accuracy in dimension of an injection-molded disc. The stamper is made of nickel, and the face which is contacted with the stamper mounting face of the mold is finished in a mirror face state.
At the time of injection molding by the above-described resin molding mold, a molten resin at a high temperature is injected into the cavity and thus a high pressure applied thereto, whereby the stamper repeats expansion and shrinkage due to the high temperature and high pressure in each shot to rub against the surface of the mold forming the cavity, thereby wearing the surface. Once the stamper mounting face of the mold is worn out, the stamper is susceptible to flaw and fracture.
Though the stamper is replaced with another in accordance with a disc to be molded, the mold is commonly used and costly, and thus it is desired to be used for a long term.
To this end, it is proposed that a hard layer composed of a material which is higher in hardness and better in resistance-to-wear than the material of a mold forming a stamper mounting face, for example, a thin film made of titanium carbide (TiC), silicon carbide (SiC), titanium nitride (TiN), or the like is formed on the stamper mounting face in an optical disc fabricating mold, for example, in the aforesaid Japanese Patent Laid-open Publication No. 62-267937.
Further, in Japanese Patent Laid-open Publication No. 1-234214 (JP, 1-234214, A), it is proposed that a part supporting a stamper of the surface forming a cavity for the disc molding mold as described above is coated with a diamond-like thin film, thereby substantially improving the resistance-to-wear and the low friction property to substantially increase useful life of the stamper and mold.
The formation, as the former, of a hard layer composed of a material which is higher in hardness and better in resistance-to-wear than the material of the stamper mounting face on the stamper mounting face of the mold surface forming the cavity of the resin molding mold is effective in enhancing the resistance-to-wear of the stamper mounting face of the molding mold and increasing useful life of the mold, but it is not sufficient only to form titanium carbide (TiC), silicon carbide (SiC), titanium nitride (TiN), or the like as the hard film.
Moreover, it is expected that by applying a diamond-like thin film (a diamond-like carbon film: abbreviated as a DLC film), as the latter, the resistance-to-wear of the stamper mounting face of the molding mold is drastically enhanced and the friction resistance with the stamper is remarkably decreased to substantially increase useful life of the stamper and mold.
However, it was found that the adhesion of the DLC film to the mold surface is weak if the DLC film is formed directly on the surface forming the cavity of the mold made of steel for the resin molding mold, bringing about problems that the DLC film peels off upon performance of polishing and lapping in order to finish the surface of the formed DLC film in a mirror face, and that the DLC film peels off by internal stress or the like during use.
The present invention is made to solve the above problems and its object is to form a DLC film which is a hard film on a surface forming a cavity of a resin molding mold at least at a part to be contacted with a stamper with strong adhesion not to easily peel off, thereby drastically increasing useful life of the resin molding mold and also increasing useful life of the stamper without causing damage thereto.
To attain the above object, this invention provides a mold for molding a resin structured as follows and a method of forming a hard film on the mold for molding a resin.
That is, a mold for molding a resin according to this invention is composed of a fixed mold and a moving mold and used with a stamper being attached to a surface of the mold forming a cavity when both molds are closed, wherein a diamond-like carbon film which is a hard film is formed on the surface of the mold at least at a part to be contacted with the stamper with an intermediate layer for enhancing adhesion strength to the surface of the mold interposed in-between.
The intermediate layer may have a single-layer structure made of any one of silicon, tungsten, titanium carbide, silicon carbide, and chromium carbide.
Further, it is more suitable that the intermediate layer has a double-layer structure composed of a lower layer made primarily of chromium or titanium and an upper layer made primarily of silicon or germanium.
Alternatively, the intermediate layer may have a double-layer structure composed of a lower layer made primarily of titanium and an upper layer made primarily of any one of tungsten, tungsten carbide, silicon carbide, and titanium carbide.
Furthermore, it is also suitable that the intermediate layer has a triple-layer structure composed of a lower layer made primarily of titanium, a middle layer made primarily of titanium carbide or silicon carbide, and an upper layer made primarily of carbon.
It is preferable that the diamond-like carbon film formed on the surface of the mold at the part to be contacted with the stamper with the intermediate layer interposed in-between has a surface roughness of 0.2 xcexcM to 0.02 xcexcm.
A method of forming a hard film on a mold for molding a resin according to this invention is a method of forming a hard film on a mold for molding resin composed of a fixed mold and a moving mold and used with a stamper being attached to a surface of the mold forming a cavity when both molds are closed, and characterized by comprising:
a step of disposing the mold with the surface thereof to which the stamper is attached having been cleaned in a vacuum chamber and evacuating it;
an intermediate layer forming step of feeding argon into the evacuated vacuum chamber and ionizing it to form an intermediate layer on the surface of the mold at least at a part to be contacted with the stamper by sputtering process with any one of silicon, tungsten, titanium carbide, silicon carbide, and chromium carbide as a target;
a step of exhausting the argon in the vacuum chamber and feeding a carbon-containing gas into the vacuum chamber; and
a step of generating plasma in the vacuum chamber to form a diamond-like carbon film on the surface of the intermediate layer by plasma CVD process.
It is more preferable to form a double-layered intermediate layer by carrying out, in place of the above intermediate layer forming step,
the first intermediate layer forming step of feeding argon into the evacuated vacuum chamber and ionizing it to form a lower layer of an intermediate layer made primarily of chromium or titanium on the surface of the mold at least at a part to be contacted with the stamper by sputtering process with chromium or titanium as a target; and
following the above step, the second intermediate layer forming step of forming an upper layer of the intermediate layer made primarily of silicon or germanium on the lower layer by sputtering process with silicon or germanium as a target.
Alternatively, it is also suitable to form a double-layered intermediate layer by carrying out, in place of the above intermediate layer forming step,
the first intermediate layer forming step of feeding argon into the evacuated vacuum chamber and ionizing it to form a lower layer of an intermediate layer made primarily of titanium on the surface of the mold at least at a part to be contacted with the stamper by sputtering process with titanium as a target; and
following the above step, the second intermediate layer forming step of forming an upper layer of the intermediate layer made primarily of tungsten on the lower layer by sputtering process with tungsten as a target.
Moreover, it is also suitable to form a double-layered intermediate layer by carrying out
the first intermediate layer forming step of feeding argon into the evacuated vacuum chamber and ionizing it to form a lower layer of an intermediate layer made primarily of titanium on the surface of the mold at least at a part to be contacted with the stamper by sputtering process with titanium as a target; and
following the above step, the second intermediate layer forming step of feeding a carbon-containing gas into the vacuum chamber to form an upper layer of the intermediate layer made primarily of tungsten carbide or silicon carbide on the lower layer by reactive sputtering process with tungsten or silicon as a target.
Furthermore, it is also suitable to form a triple-layered intermediate layer by carrying out
the first intermediate layer forming step of feeding argon into the evacuated vacuum chamber and ionizing it to form a lower layer of an intermediate layer made primarily of titanium on the surface of the mold at least at a part to be contacted with the stamper by sputtering process with titanium as a target;
following the above step, the second intermediate layer forming step of feeding a carbon-containing gas into the vacuum chamber to form a middle layer of the intermediate layer made primarily of titanium carbide or silicon carbide on the lower layer by reactive sputtering process with titanium or silicon as a target; and
following the above step, the third intermediate layer forming step of forming an upper layer made primarily of carbon on the middle layer by gradually reducing a sputtering amount of titanium or silicon as the target.
In the method of forming a hard film on a mold for molding a resin, it is desirable to carry out, after the step of forming the diamond-like carbon film, the step of finish grinding the surface of the diamond-like carbon film formed in the step by polishing and lapping.
It is suitable that the polishing and lapping in the step of finish grinding are performed using a diamond paste and an alumina paste with particle diameters of diamond and alumina ranging from 0.1 xcexcm to 4 xcexcm.